In the production of microelectronic and micromechanical devices, such as semiconductors, memory, processors, and controllers, among others, a mask is used. The mask is placed over a semiconductor wafer to expose or shield different portions of the wafer from light, or some other element. The exposed wafer is then processed with etching, deposition and other processes to produce the features of the various semiconductors in the wafer that make up the finished product.
The masks are typically but not exclusively made up of a quartz plate with a pattern of chrome lines and blocks. Masks can also be made up of reflective mask technology for extreme ultraviolet wavelengths. The quartz allows light to pass and the metal lines reflect light. Different materials are used in different applications and masks may also have portions that change the phase of some of the light in order to control interference or diffraction effects. The masks are designed using computer design programs that derive an aerial view or image of the printed wafer based on the electronic circuitry that is to be built on the wafer. The mask is designed to produce this aerial image on the wafer based on using a particular set of photolithography equipment. In other words, the mask must be designed so that when a particular wavelength of light at a particular distance is directed to a wafer through a particular set of optics and the mask, the desired pattern will be illuminated with the desired intensity on the wafer. The complexity of each mask used to make a chip reflects the complexity of that chip.
In order to enhance the accuracy and the resolution of the pattern that results on the wafer. A variety of different optimization techniques are typically applied to the mask. These techniques include optical proximity correction (OPC), off-axis illumination (OAI), attenuated phase shifted mask (APSM) enhanced lithography, embedded phase shifted mask (EPSM) lithography, extreme ultraviolet (EUV) & X-ray reflective mask technology, and other techniques. These techniques are optimized for an expected range of variations in the parameters of the printing process (focus, intensity, chemistry, wafer composition, etc.). (The combination of these variations of the parameters of the printing process is sometimes referred to as the process window.)
OPC and other techniques may be tailored to particular fabrication processes and process windows by applying measured parameters of the process. The scattered light point spread function (PSF) may be used by OPC, for example, to improve the results of OPC for a particular process window. The PSF of the scattered light of a scanner for optical proximity correction (OPC) indicates the amount of scattered light and the range of travel distances of the scattered light. The PSF impacts the control of critical dimensions (CD) in a microelectronic circuit or device.
The point spread function (PSF) of scattered light on a lithography exposure tool (scanner) is determined by printing patterns on a wafer and then measuring the results. The mask used to print the features has features with a range of dimensions so that the scattering with features of different shapes and at different dimensions can be compared. Based on the resulting printed wafer, the change in a monitoring feature's critical dimensions (CD) as printed on the wafer can be measured.
The CD of a monitoring feature is varied by modifying the layout environment around it. The layout modifications cause variations in the scattered light intensity. PSF is measured in at least two different ways. In one approach, a one dimensional transparency edge is with monitoring features located at different distances from the one dimensional edge on the mask. Another approach uses two dimensional rectangular shaped transparency edges and the monitoring features are placed relative to the rectangular edges. There are a large variety layouts for the two dimensional rectangular methods which include the monitoring feature in the center of the rectangular pattern.